Why Water droplets confuse your smart phone touch screen?

Touchscreen locates your finger on the grid by measuring how much the charge drops between two intersecting electrodes, a process called “mutual capacitance.”
The problem is that drops of sweat or rain can reduce the charge too by providing another conduit between the electrodes. Thankfully, over the past few years, touchscreen engineers have solved the water problem by drawing on a different mode of touch sensing called “self-capacitance.”
Instead of measuring the charge across pairs of electrodes, the touchscreen measures the increase in charge between an individual electrode on the screen and the ground you’re standing on. Because water droplets aren’t grounded, the phone’s firmware is better able to ignore them.
But it is also not fully successful because it won't work during multipoint touch operations like pinch, zoom etc.